1. Field of the Invention
The present invention relates to a diffractive optical modulator for modulating a light intensity and a method for producing the same; an infrared sensor including such a diffractive optical modulator and a method for producing the same; and a display device including such a diffractive optical modulator.
2. Description of the Related Art
A diffractive optical modulator modulates the intensity of incoming light by using the diffraction. A conventional diffractive optical modulator is disclosed, for example, in O. Solgaard et al., "Deformable Grating Optical Modulator," Optics Letters, Vol. 17, No. 9, May 1, 1992. The diffractive optical modulator disclosed in this article modulates the light intensity by using the diffraction effects of light. This modulator can be produced with a reduced size by an IC process, making such a modulator suitable for mass-producing.
The diffractive optical modulator disclosed in this article includes: a silicon substrate; a spacer (or silicon oxide film) formed in a peripheral region on the silicon substrate; and a grating consisting of a plurality of fine dielectric beams (or silicon-rich silicon nitride films), both ends of which are supported by the spacer. A reflection film functioning also as an electrode is provided on the upper surface of the grating. When a voltage is applied between the reflection film and the silicon substrate, an electrostatic force (or a Coulomb's force) is generated therebetween, so that the grating is deflected. As a result, the lower surface of the deflected grating comes into contact with the upper surface of the silicon substrate. Since the distance between the reflection film provided on the upper surface of the grating and the reflection film provided on the upper surface of the silicon substrate is varied in accordance with the application of a voltage, the diffraction efficiency is also varied.
In a conventional diffractive optical modulator, the beams are made of a dielectric material, and the modulator is driven by applying a voltage between the reflection film provided on the upper surface of the beams and the substrate. Accordingly, in the case of modulating light having a long wavelength such as infrared light, the distance between the upper and the lower electrodes becomes long, so that the driving voltage is required to be adversely high. In addition, the beams are formed by nitride films, and therefore a strong residual tensile stress is caused in the films. In general, the residual stress in a nitride film is almost as large as 1 GPa. In the above-mentioned conventional example, the beams are formed by silicon-rich nitride films, so that the residual stress is reduced to about 200 MPa. However, it is very difficult to reduce the tensile stress to a small one in accordance with such a method for reducing the stress, and the uniformity of the film is also degraded. Moreover, the spacer layer is formed by a silicon oxide film, and then removed by a wet etching (W/E) process. When the grating is dried after being rinsed, the surface tension of the rinsing liquid adversely causes the sticking of the beams onto the surface of the substrate. In order to solve such problems, a method in which protrusions provided on the lower surfaces of the beams prevent the sticking, or a so-called freeze drying method in which the beams are frozen, for example, in pure water after being rinsed and the frozen pure water is sublimated under vacuum has been used. However, both the methods disadvantageously complicate the production process.